Objectives: To measure the size of the heart, caudal vena cava and right pulmonary artery (cats) or right caudal lobar artery (dogs) using the vertebral scale method in heartworm-infected cats and dogs and compare the results with reference values.

Materials and methods: Standardized measurements using a previously published method were made using lateral, dorsoventral and/or ventrodorsal thoracic radiographs.

Results: On lateral radiographs, the mean VHS for the heartworm-infected group was significantly larger than the reference value for both cats and dogs (cats p = 0.009, dogs p < 0.001). On lateral radiographs of both infected cats and dogs, there was a significant correlation between mean diameter of the caudal vena cava and VHS (cats p = 0.002, r = 0.59; dogs p = 0.012, r = 0.53). The mean ratio of the width of the right pulmonary artery (cats) or right caudal lobar artery (dogs) to the width of rib 9 was higher than reference values for each species.

Conclusions: Heartworm-infected cats and dogs often have an enlarged cardiac silhouette on thoracic radiographs. The VHS method, measurement of the caudal vena cava expressed in vertebral lengths, and the calculation of the ratio of the right pulmonary or caudal lobar artery width to rib 9 width are all useful tools to monitor the natural history of the disease and aid in staging for therapy of heartworm disease.